What is "floated" linear voltage regulator?

[001]

Hi!

I hear something about "floated" linear voltage regulator. But what it is actually? 

[capt bullshot]

Difficult to answer without context.
It might refer to some regulator circuitry that has an pass element (usually some kind of transistor) and a regulation circuit built around the pass element. The regulation circuits "GND" potential is tied to the pass elements output (e.g. the emitter of an BJT), and has a floating supply independent from its input and output. Output voltage sensing is from output "GND" (that isn't identical to the regulator "GND") to output voltage.
This is a common style for many lab power supplies, one main advantage is that the regulator circuitry doesn't have to operate from the main supply. Only the pass element has to stand the input to output voltage difference. So common +/- 12V operated OP-Amps can be used to build e.g. a 0 ... 200V supply.

[MiDi]

LM317/337 are examples, they are specified for maximum voltage between input and output and have no reference to GND.
For example they are suitable for 200V input and 180V output.
But care is needed that you ALWAYS keep input and output difference inside specs, this is a problem on startup and if output could have short circuit.

[SiliconWizard]

I have heard of "floated wood", but floated regulators? Hmm.
 

[MiDi]

I have heard of "floated wood", but floated regulators? Hmm.
 

You know what he means 
-> floating

[001]

Thanx a lot 

Can You post some good discrete circuit?

[ogden]

Can You post some good discrete circuit?

It's in the LM317 datasheet. Look for adjustable regulator circuit.

[001]

Can You post some good discrete circuit?

It's in the LM317 datasheet. Look for adjustable regulator circuit.

LM317 is integrated circuit 

[ogden]

Can You post some good discrete circuit?

It's in the LM317 datasheet. Look for adjustable regulator circuit.

LM317 is integrated circuit 

So what?

[schmitt trigger]

I believe that perhaps the OP is referring to supplies which are galvanically isolated from ground or at least from the main circuit’s common.

The simplest of course is a battery.

[001]

Can You post some good discrete circuit?

It's in the LM317 datasheet. Look for adjustable regulator circuit.

LM317 is integrated circuit 

So what?

I`m looking for basic discrete design

[ogden]

LM317 is integrated circuit 

So what?

I`m looking for basic discrete design

LM317 is basic design and it shall be considered as "discrete enough" Anything else will be much more complex, most likely will require opamp and (shunt) reference - both discrete components You can try to search for "high voltage discrete linear supply circuit" but I am afraid that you can find such only in oldskoll electronics/radio magazines. Good luck, you will need it: https://www.americanradiohistory.com/

[001]

LM317 is integrated circuit 

So what?

I`m looking for basic discrete design

LM317 is basic design and it shall be considered as "discrete enough" Anything else will be much more complex, most likely will require opamp and (shunt) reference - both discrete components You can try to search for "high voltage discrete linear supply circuit" but I am afraid that you can find such only in oldskoll electronics/radio magazines. Good luck, you will need it: https://www.americanradiohistory.com/

yea
oldshcool is ok too
Only transistors and resistors 


oh I see what You from former USSR
Can You say about cold war gear
Someone tells it will be work to the end of time
Is it truth?

[ebastler]

yea
oldshcool is ok too
Only transistors and resistors 


oh I see what You from former USSR
Can You say about cold war gear
Someone tells it will be work to the end of time
Is it truth?

You just posted the Statron manuals, including schematics, in that other thread. Aren't those discrete enough for you? 

Also, please be mindful with your USSR and cold war references. The Baltic republics (and quite a few other countries too) did not particularly enjoy being part of the USSR, and their citizens may not enjoy being characterized as "former USSR" as the main identification. And, come to think of it -- why not ask a few Americans about "cold war gear"? If I recall correctly, there were two sides in that "war"...

On a more general note, it leaves a bad aftertaste for me when you talk about fellow forum users' countries of origin, but have apparently chosen to mask your own. Why don't you tell us where you are from?

[001]

yea
oldshcool is ok too
Only transistors and resistors 


oh I see what You from former USSR
Can You say about cold war gear
Someone tells it will be work to the end of time
Is it truth?

You just posted the Statron manuals, including schematics, in that other thread. Aren't those discrete enough for you? 

Also, please be mindful with your USSR and cold war references. The Baltic republics (and quite a few other countries too) did not particularly enjoy being part of the USSR, and their citizens may not enjoy being characterized as "former USSR" as the main identification. And, come to think of it -- why not ask a few Americans about "cold war gear"? If I recall correctly, there were two sides in that "war"...

On a more general note, it leaves a bad aftertaste for me when you talk about fellow forum users' countries of origin, but have apparently chosen to mask your own. Why don't you tell us where you are from?

Yea
I see  Statron is not overengineered
Its nice 
Is all easteuropean gear are buided this way too? I want to know more

[Vtile]

Yes. All eastern euro stuff are the same. The electrical knowledge were and still is very limited especially eastern Europe since 16th century, since none of the great electrical minds dared to go there and the ones there was escaped if they could.

[schmitt trigger]

The interesting thing about Statron, is that they are still in business, building test equipment.

[001]

The interesting thing about Statron, is that they are still in business, building test equipment.

It will be interesting to see their actual schematics vs legacy

[ogden]

Is all easteuropean gear are buided this way too? I want to know more

During occupation Baltics were at the technological level of USSR, now are catching-up. Just one example - SAF spectrum analyzers

[Wolfgang]

Hi,

thats easy. A floating regulator is a regulator which has no direct ground connection.

Conterexample: 7805. Input-Ground-Output. Not floating.
Floating: LM317: Input-Output-Adjust. This regulator never sees "ground", it floats.

The LM723 can be used in floating and non-floating configurations, see datasheet.

[001]

Hi,

thats easy. A floating regulator is a regulator which has no direct ground connection.

Thanx
Can You comment this eastern europe schematic?

[precaud]

Floating: LM317: Input-Output-Adjust. This regulator never sees "ground", it floats.

So how does a floating regulator create/have a low impedance to ground if it never references to it?

[001]

Floating: LM317: Input-Output-Adjust. This regulator never sees "ground", it floats.

So how does a floating regulator create/have a low impedance to ground if it never references to it?

I don`t understand it too

[xavier60]

Hi,

thats easy. A floating regulator is a regulator which has no direct ground connection.

Thanx
Can You comment this eastern europe schematic?

It needs to be mentioned that this design uses the mains switch and transformer's primary winding to discharge the large capacitor.
Very hazardous!

[xavier60]

Floating: LM317: Input-Output-Adjust. This regulator never sees "ground", it floats.

So how does a floating regulator create/have a low impedance to ground if it never references to it?

There will always be a feedback divider to sense the voltage of the - output terminal with respect to the + output terminal.

[001]

Floating: LM317: Input-Output-Adjust. This regulator never sees "ground", it floats.

So how does a floating regulator create/have a low impedance to ground if it never references to it?

There will always be a feedback divider to sense the voltage of the - output terminal with respect to the + output terminal.

Thanx!
Did You recive my letter about Yours restoration project?

[Wolfgang]

Hi,

thats easy. A floating regulator is a regulator which has no direct ground connection.

Thanx
Can You comment this eastern europe schematic?

It needs to be mentioned that this design uses the mains switch and transformer's primary winding to discharge the large capacitor.
Very hazardous!

This is indeed a floating design. The top part contains an auxiliary PSU centered at the positive output voltage and feeding a two-stage long tailed pair error amp design for voltage and current. The diodes in the top middle define if the PSU is running in CV or CC mode (priority circuit). The string of pass transitors is darlington driven from this voltage. Sensing voltage and current is from the part top right with a Zener diode and resistive dividers.

[xavier60]

Floating: LM317: Input-Output-Adjust. This regulator never sees "ground", it floats.

So how does a floating regulator create/have a low impedance to ground if it never references to it?

There will always be a feedback divider to sense the voltage of the - output terminal with respect to the + output terminal.

Thanx!
Did You recive my letter about Yours restoration project?

No, I don't understand. I only posted about the bench supply that I made lately which is a floating type.

[001]

o, I don't understand. I only posted about the bench supply that I made lately which is a floating type.

Thanx! Sorry, it`s my mistake. I mean other Xavier   

[ArthurDent]

"So how does a floating regulator create/have a low impedance to ground if it never references to it?"

There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts.

[001]

"So how does a floating regulator create/have a low impedance to ground if it never references to it?"

There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts.

Thanx! Why this design is not popular at the West countries? (all found schematics are from Eastern Germany, Hungary and Bulgarian Republic)

[ArthurDent]

Almost if not all the bench supplies I have have floating outputs. To have one output terminal grounded would severely limit the usefulness of the supply. A supply built into a stereo or some other piece of equipment is different because it isn't designed for general use and more often than not will have one output terminal grounded.

[xavier60]

Almost if not all the bench supplies I have have floating outputs. To have one output terminal grounded would severely limit the usefulness of the supply. A supply built into a stereo or some other piece of equipment is different because it isn't designed for general use and more often than not will have one output terminal grounded.

It would be better to call that "isolated" supply.

[xavier60]

Some of us might be talking about different things. "floating type" to me means that the control circuitry has its ground reference connected to the + output terminal rather than the - terminal. The control circuitry floats up and down with the + output  voltage.

[001]

Some of us might be talking about different things. "floating type" to me means that the control circuitry has its ground reference connected to the + output terminal rather than the - terminal. The control circuitry floats up and down with the + output  voltage.

Yea
This topology  is interesting and new for me
Where I can read about? Some hanbook and classical schematic?

[xavier60]

Some of us might be talking about different things. "floating type" to me means that the control circuitry has its ground reference connected to the + output terminal rather than the - terminal. The control circuitry floats up and down with the + output  voltage.

Yea
This topology  is interesting and new for me
Where I can read about? Some hanbook and classical schematic?

You have been asking about this topic for a long time. What is your goal?
I did a search and confirmed that "Floating regulator" is a thing, http://www.circuitstoday.com/floating-regulator
The LM723 is powered from the voltage difference between Vin and Vout. This is not a good idea in practice as the regulator will have a higher than usual dropout voltage.
The important point is that the only connection to the ground symbol is the voltage sense resistor.

[001]

the LM723 is powered from the voltage difference between Vin and Vout. This is not a good idea in practice as the regulator will have a higher than usual dropout voltage.
The important point is that the only connection to the ground symbol is the voltage sense resistor.

723 sucks since it didn`t use stabile reference and no way to intelegent current control.

[capt bullshot]

If you can read German, here's some schematics and documentation of Heinzinger "Transistornetzteile" (Made in Western Germany  :
https://cb.wunderkis.de/wk-pub/heinzinger/

[spec]

Just been through this thread. Here is my 2 cents worth.

Floating is a general term which means not connected to a reference point of some sort, normally 0V.

But when it comes to the special use of floating to describe a type of power supply architecture, floating means that the power supply circuitry does not see the full input or output voltage across its terminals. All a floating regulator sees, in essence, is the difference between its input and output voltages.

Examples of floating supplies are practically all three terminal regulators, both positive and negative. For example the ubiquitous LM317 has a maximum allowable voltage of 40V between its input terminal and output terminal. But an LM317 can be made to supply 200V say because it is floating.

In the above case the raw supply might be 220V and, as the output voltage is 200V, the difference between the LM317 input terminal and output terminal is 220V-200V =20V, which is well within the 40V limit of an LM317.

Because the LM317 is a floating supply, its sense terminal is always 1.25 volts more negative than than its output terminal. That 1.25V, which is like an accurate Zener diode, is the voltage reference for the feedback loop that stabilizes the output voltage.

The term floating is really a misnomer, because all voltage regulators are connected, one way or another, to 0V: they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the LM317 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V. 

[001]

If you can read German, here's some schematics and documentation of Heinzinger "Transistornetzteile" (Made in Western Germany  :
https://cb.wunderkis.de/wk-pub/heinzinger/

Awesome link. But all pdfs are unawailable now (It says: "You don't have permission to access /wk-pub/heinzinger/0390 30258 LNG 50-4.pdf on this server."  )
Can You share it here?

[001]

The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V.

Thanx a lot! Can You tell me what reference is used in floating designs? How it "feels" ground potential?

[spec]

No probs.

I will put together a simple explanation and include a discrete circuit which hopefully will illustrate the way that the feed back works in a floating PSU.

You can greatly simplify electronics by forming simple models of circuit elements. One simple model, which has great relevance to PSUs is this: "An opamp will do all it can to make its non-inverting and inverting inputs the same.  And the only thing it can do is to change its output voltage."  That simple concept helps you understand what an opamp is doing in most circuits without any heavy math or circuit analysis.

There are simple models for all elementary components: resistors, capacitors, inductors, transformers, diodes, BJTs, MOSFETs, and opamps.

And here is a simple model of a PSU: "A PSU is an opamp with a high output current capability."

[capt bullshot]

If you can read German, here's some schematics and documentation of Heinzinger "Transistornetzteile" (Made in Western Germany  :
https://cb.wunderkis.de/wk-pub/heinzinger/

Awesome link. But all pdfs are unawailable now (It says: "You don't have permission to access /wk-pub/heinzinger/0390 30258 LNG 50-4.pdf on this server."  )
Can You share it here?

Seems you'll have to turn on the Referer on your browser, or try another browser. I'm using Firefox, and it works for me. This is a configuration I made to my webserver to keep search engines and other scrapers away, maybe it blocks you unwanted. If your browser provides the correct referer, you should be able to get the files.
Forget the above, and try again, apparently I didn't set the access rights to the files correctly, so the server couldn't access them at atll.

[not1xor1]

Here is an example of a voltage regulator (current regulation omitted for clarity).
BTW probably it would be more appropriate to define it bootstrapped rather than floating.

Also, these pictures show the advantage of having variable Vref vs. variable feedback.


variable feedback


variable voltage reference

[capt bullshot]

BTW probably it would be more appropriate to define it bootstrapped rather than floating.

"Bootstrapped" usually refers to a supply voltage above the maximum voltage level in the system, stored in a capacitor that gets charged while the output swings lower. It's a classic technique in some audio amplifiers, and often used in MOSFET driver circuitries. Bootstrapping won't work on a stable DC output (which is desired for a lab power supply).

[xavier60]

The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V.

Thanx a lot! Can You tell me what reference is used in floating designs? How it "feels" ground potential?

You have been getting answers. It would help if we know how much knowledge you have. Do you understand op-amps and resistor dividers?

[Zero999]

"So how does a floating regulator create/have a low impedance to ground if it never references to it?"

There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts.

That's true, but with a DC power supply we always need a very low DC resistance. The the linear regulator is too slow, then the output impedance will rise with increasing frequency, which is why a decoupling capacitor is used.

The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V.

Thanx a lot! Can You tell me what reference is used in floating designs? How it "feels" ground potential?

It doesn't feel ground potential, because no terminal in the floating regulator is directly connected to ground.

Look at the simplified schematic of an LM317. It consists as an op-amp with it's negative supply rail connected to the output terminal. The inverting input is also connected to the output and the non-inverting input is connected to the adjust pin, via a 1.25V voltage reference. A tiny 50µA current source biases the reference and op-amp's non-inverting input.

As with any op-amp circuit, the op-amp adjusts its output, turning the output transistor more on or off to ensure the voltage at both its inputs is the same. Because the non-inverting input is connected to the adjust pin via 1.25V reference, the voltage at the adjust pin will be held at 1.25V below the output.



Now let's look at the basic application circuit for the LM317.


The output is connected to the adjust pin via a R1. Because the voltage between the output and adjust pin is fixed 1.25V, the current through R1 will also be constant. R2 is in series with R1, thus the current through it will also be constant. A tiny 50µA bias current flows out of the adjust pin and through R2, but it's normally small enough to ignore. Hopefully from this statement, you can derived the formula for calculating the output voltage, using Ohm's law.

The main advantage of this configuration is the op-amp's power supply is equal to the difference between the input and output voltages. This means the input-output differential limits the maximum supply voltage, rather than the op-amp or pass transistor's voltage rating. Note that if the protection circuitry (over current/temperature) is triggered, the output transistor will start to turn off, thus lowering the output voltage and increasing the total power supply voltage to the op-amp, which will be damaged if the supply voltage exceeds its maximum rating. Thus, if the protection circuitry is required the total supply voltage must be within the regulator's maximum rating, unless additional parts are added to limit it.

The downsides are the 50µA bias current flows out of the adjust pin which will cause the voltage across R2 and therefore the output voltage to be slightly higher, than the basic calculation suggests but that can be easily be calculated by adding R2*50*10^-6. The op-amp's supply current flows also through the load, so a minimum current is required to ensure the op-amp has sufficient power to operate, but that can be ensured by making R1 low enough to pass the minimum current with no load.

It's possible to build your own floating regulator, from discrete parts, but it's not practical. You'll need to design a discrete op-amp with: a common mode range which includes the negative rail, a wide supply voltage (<3V to ensure the dropout voltage isn't too high, up to the maximum expected input-output voltage) and capable of outputting sufficient current to the driver transistors.  Discrete transistors aren't matched, so the performance will be inferior to an IC design and over-temperature protection is virtually impossible with a discrete design.

[schmitt trigger]

It needs to be mentioned that this design uses the mains switch and transformer's primary winding to discharge the large capacitor.
Very hazardous!

 
I hadn't noticed that!, thanks for pointing it out.
They saved the cost of a discharge resistor. But the more complex DPDT switch instead of a DPST would offset that, don't you think?

[Zero999]

It needs to be mentioned that this design uses the mains switch and transformer's primary winding to discharge the large capacitor.
Very hazardous!

 
I hadn't noticed that!, thanks for pointing it out.
They saved the cost of a discharge resistor. But the more complex DPDT switch instead of a DPST would offset that, don't you think?

There's also the potential for it to go wrong, as hinted above. The DPDT switch would have to be rated to give full isolation from the mains.

I don't think it's about cost, but discharging the capacitor quickly, without having to use a low value, high power resistor which would get very hot. A safer solution would be to use a suitable resistor and another set of properly isolated switch contacts or a small relay.

[ArthurDent]

"So how does a floating regulator create/have a low impedance to ground if it never references to it?"

There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts.

That's true, but with a DC power supply we always need a very low DC resistance. The the linear regulator is too slow, then the output impedance will rise with increasing frequency, which is why a decoupling capacitor is used.

The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V.

That's not what I said at all. I said "high resistance to ground", not impedance. I also said "to ground", not to 0 volts. The output impedance between the output terminals of a power supply should be low but we generally want the resistance to ground of a bench supply to be high or infinite. This gives us the ability to wire one supply in series with another without shorts and this is what is done with dual supplies where they can be switched for either series or parallel operation. Supplies floating with respect to ground allow us to do this or a single output supply to be wired either + or - with respect to ground.

If you look at the schematic 001 posted in post #20 to show this is what he meant. You will see this in the lower right hand corner of the schematic as shown below. The circuit has high resistance but low impedance. With a capacitor to ground the 'impedance' decreases with increasing frequency.

[David Hess]

For regulators which can be configured either way, it means that the regulator's quiescent current flows into the output instead of to ground; the output becomes "common" for purposes of the circuit design.  This allows voltages in excess of the regulator's breakdown voltage to be controlled as long as the maximum input-to-output voltage is not exceeded.  The 723 application notes show examples of both.

Contrast the LM7805 which is not a floating regulator and has its quiescent current flow out through its "common" pin with the LM317 which is a floating regulator and has its quiescent current flow out through its output pin.

[Zero999]

"So how does a floating regulator create/have a low impedance to ground if it never references to it?"

There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts.

That's true, but with a DC power supply we always need a very low DC resistance. The the linear regulator is too slow, then the output impedance will rise with increasing frequency, which is why a decoupling capacitor is used.

The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V.

That's not what I said at all. I said "high resistance to ground", not impedance. I also said "to ground", not to 0 volts. The output impedance between the output terminals of a power supply should be low but we generally want the resistance to ground of a bench supply to be high or infinite. This gives us the ability to wire one supply in series with another without shorts and this is what is done with dual supplies where they can be switched for either series or parallel operation. Supplies floating with respect to ground allow us to do this or a single output supply to be wired either + or - with respect to ground.

If you look at the schematic 001 posted in post #20 to show this is what he meant. You will see this in the lower right hand corner of the schematic as shown below. The circuit has high resistance but low impedance. With a capacitor to ground the 'impedance' decreases with increasing frequency.

You appear to be confused about the context of the word "ground" in the title of the thread. For the purposes of discussing an electrical circuit, the word ground refers to 0V. Whether it is connected to the physical earth or not is immaterial. Ground is purely a reference point in circuit from where all voltages are measured from.

A floating linear regulator is a circuit who's common rail is at a different  potential to that of the rest of the circuit. The LM317 is a floating linear regulator because it has no ground pin. The adjust pin connects to the centre of a potential divider, thus it floats at a higher voltage than the circuit's ground or reference. The LM7805 is not a floating regulator because its ground pin is normally connected to the same reference as the rest of the circuit.

Yes, any bench power supply should be isolated or floating with respect to mains ground, but this has absolutely nothing to do with the definition of a floating linear regulator, such as the LM317 and is completely off-topic.

[not1xor1]

BTW probably it would be more appropriate to define it bootstrapped rather than floating.

"Bootstrapped" usually refers to a supply voltage above the maximum voltage level in the system, stored in a capacitor that gets charged while the output swings lower. It's a classic technique in some audio amplifiers, and often used in MOSFET driver circuitries. Bootstrapping won't work on a stable DC output (which is desired for a lab power supply).

In this case it is the control circuit (opamp) supply which is bootstrapped by the positive output voltage (nothing forbids to use a PNP pass transistor and bootstrap to the negative rail).
Usually that is achieved by a separate opamp supply whose 0V rail is tied to the positive output rail, but you might use an higher voltage rail and separate regulators to get 2 constant voltage positive/negative rails tied to the output rail.

addendum 
I apologize. I think you're right and although the terms "floated" and "bootstrapped" might be interchangeable in this case it might be indeed more appropriate to define the circuit as floating.

[not1xor1]

Here is a circuit that replicates LM317 topology.
This is a proof of concept, not a real circuit, so I used a voltage source rather than a real voltage reference.

Unlike what I foolishly wrote a few days ago in another thread, the adjust resistor does change the loop gain.
So the phase margin of a real LM317 is probably at its worst when the adjust pin is directly attached to the circuit ground (pls. notice the difference between ground and earth).

The effect of the noise suppressing capacitor (then one in parallel to the adjust resistor) is to keep high frequency loop gain constant (see the bottom picture), so regardless of the value of the adjust resistor the phase margin is always at its worst.



here are output voltage and adjust resistor current when stepping the resistor from 1mΩ to 1.2kΩ



here is the AC analysys. The phase margin of this circuit is just 47° when the adjust pin is connected to ground (adjust resistor 1mΩ)



I highlighted the last step (1.2kΩ resistor) where the phase margin is about 80°



This is the effect of 10µF capacitor in parallel to the adjust resistor.

[David Hess]

Unlike what I foolishly wrote a few days ago in another thread, the adjust resistor does change the loop gain.
So the phase margin of a real LM317 is probably at its worst when the adjust pin is directly attached to the circuit ground (pls. notice the difference between ground and earth).

That is right and why the output capacitor stability requirements are greater when the gain resistor is bypassed to ground for lower noise.

[spec]

Wow, a lot of posts since I was here last.

Attached is the schematic promised in reply #41 that shows simplified circuits for a 'conventional' 0V referenced PSU and a floating PSU. In both cases the feedback is shown. Notice that in both circuits the opamps non-inverting and inverting inputs are at precisely the same voltages. With precision resistors the circuits could form precision voltage references, with an output current capability of around 40ma, as subsequently mentioned in reply #57 by Silicon Wizard.

I have indicated actual components as a guide, so you can check details on the appropriate datasheets (links below). Note that the 1V25 reference diode runs at only a few micro amps. This has no significance in schematic #1, because the reference diode current just flows to 0V, but in schematic #2 the voltage reference current flows down the feedback resistor chain so it is important to keep the voltage reference current small compared to the tail current, which is a whopping 12.5mA. In some three terminal regulators the voltage reference current is made constant though.

Once you have digested these two circuits, you may like to have a look at current referenced floating supplies.

Also the opamp is a rail to rail input/output (RRIO) type, which means that its output can swing to its positive and negative rails. It also means that it can function as an opamp with an input voltage ranging from its negative rail to its positive rail. This characteristic has opened the way for a whole raft of circuit designs that could not be achieved with the older 741 type opamps, who's inputs and outputs could only get within a couple of volts of their negative and positive rails. It also explains why the opamp's rather odd-looking negative rail arrangement in schematic #2 works.

I have also indicated voltages in both circuits. If you have any questions just ask.

https://datasheets.maximintegrated.com/en/ds/MAX6006A-MAX6009B.pdf
http://www.ti.com/lit/ds/sbos620e/sbos620e.pdf

[spec]

"So how does a floating regulator create/have a low impedance to ground if it never references to it?"

There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts.

That's true, but with a DC power supply we always need a very low DC resistance. The the linear regulator is too slow, then the output impedance will rise with increasing frequency, which is why a decoupling capacitor is used.

The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V.

That's not what I said at all. I said "high resistance to ground", not impedance. I also said "to ground", not to 0 volts. The output impedance between the output terminals of a power supply should be low but we generally want the resistance to ground of a bench supply to be high or infinite. This gives us the ability to wire one supply in series with another without shorts and this is what is done with dual supplies where they can be switched for either series or parallel operation. Supplies floating with respect to ground allow us to do this or a single output supply to be wired either + or - with respect to ground.

If you look at the schematic 001 posted in post #20 to show this is what he meant. You will see this in the lower right hand corner of the schematic as shown below. The circuit has high resistance but low impedance. With a capacitor to ground the 'impedance' decreases with increasing frequency.

You appear to be confused about the context of the word "ground" in the title of the thread. For the purposes of discussing an electrical circuit, the word ground refers to 0V. Whether it is connected to the physical earth or not is immaterial. Ground is purely a reference point in circuit from where all voltages are measured from.

A floating linear regulator is a circuit who's common rail is at a different  potential to that of the rest of the circuit. The LM317 is a floating linear regulator because it has no ground pin. The adjust pin connects to the centre of a potential divider, thus it floats at a higher voltage than the circuit's ground or reference. The LM7805 is not a floating regulator because its ground pin is normally connected to the same reference as the rest of the circuit.

Yes, any bench power supply should be isolated or floating with respect to mains ground, but this has absolutely nothing to do with the definition of a floating linear regulator, such as the LM317 and is completely off-topic.

I do not understand what you are trying to say here. Who is confused about what?

What is the relevance of mentioning that a PSU can be isolated from the mains?

Please clarify so your view can be understood and discussed.

[SiliconWizard]

Attached is the schematic promised in Answer #41 that shows simplified circuits for a 'conventional' ground referenced PSU and a floating PSU. In both cases the feedback is shown.
(...)

I may have missed something, but the OPA192 has limited current output capability so, a PSU yes, but with something like 50 mA max. probably... if it's not further buffered. Just a thought!

[spec]

Attached is the schematic promised in Answer #41 that shows simplified circuits for a 'conventional' ground referenced PSU and a floating PSU. In both cases the feedback is shown.
(...)

I may have missed something, but the OPA192 has limited current output capability so, a PSU yes, but with something like 50 mA max. probably... if it's not further buffered. Just a thought!

No, you have not missed anything. Your observation about the current limitation is correct. There are missing functions too, like decoupling, compensation, and current limiting. But the purpose of the schematics is to illustrate the two feedback techniques which the OP was asking about. If I did a full design you would not be able to see the wood for the trees.

Just for a bit of fun, I did have a go at a 1A version with current limiting and all the twiddly bits. 

[spec]

Here is a circuit that replicates LM317 topology.
This is a proof of concept, not a real circuit, so I used a voltage source rather than a real voltage reference.

Unlike what I foolishly wrote a few days ago in another thread, the adjust resistor does change the loop gain.
So the phase margin of a real LM317 is probably at its worst when the adjust pin is directly attached to the circuit ground (pls. notice the difference between ground and earth).

The effect of the noise suppressing capacitor (then one in parallel to the adjust resistor) is to keep high frequency loop gain constant (see the bottom picture), so regardless of the value of the adjust resistor the phase margin is always at its worst.



here are output voltage and adjust resistor current when stepping the resistor from 1mΩ to 1.2kΩ



here is the AC analysys. The phase margin of this circuit is just 47° when the adjust pin is connected to ground (adjust resistor 1mΩ)



I highlighted the last step (1.2kΩ resistor) where the phase margin is about 80°



This is the effect of 10µF capacitor in parallel to the adjust resistor.

Very interesting and informative. Thanks for posting

[Zero999]

"So how does a floating regulator create/have a low impedance to ground if it never references to it?"

There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts.

That's true, but with a DC power supply we always need a very low DC resistance. The the linear regulator is too slow, then the output impedance will rise with increasing frequency, which is why a decoupling capacitor is used.

The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example.

Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V.

That's not what I said at all. I said "high resistance to ground", not impedance. I also said "to ground", not to 0 volts. The output impedance between the output terminals of a power supply should be low but we generally want the resistance to ground of a bench supply to be high or infinite. This gives us the ability to wire one supply in series with another without shorts and this is what is done with dual supplies where they can be switched for either series or parallel operation. Supplies floating with respect to ground allow us to do this or a single output supply to be wired either + or - with respect to ground.

If you look at the schematic 001 posted in post #20 to show this is what he meant. You will see this in the lower right hand corner of the schematic as shown below. The circuit has high resistance but low impedance. With a capacitor to ground the 'impedance' decreases with increasing frequency.

You appear to be confused about the context of the word "ground" in the title of the thread. For the purposes of discussing an electrical circuit, the word ground refers to 0V. Whether it is connected to the physical earth or not is immaterial. Ground is purely a reference point in circuit from where all voltages are measured from.

A floating linear regulator is a circuit who's common rail is at a different  potential to that of the rest of the circuit. The LM317 is a floating linear regulator because it has no ground pin. The adjust pin connects to the centre of a potential divider, thus it floats at a higher voltage than the circuit's ground or reference. The LM7805 is not a floating regulator because its ground pin is normally connected to the same reference as the rest of the circuit.

Yes, any bench power supply should be isolated or floating with respect to mains ground, but this has absolutely nothing to do with the definition of a floating linear regulator, such as the LM317 and is completely off-topic.

I do not understand what you are trying to say here. Who is confused about what?

What is the relevance of mentioning that a PSU can be isolated from the mains?

Please clarify so your view can be understood and discussed.

That was my point: the fact the PSU is isolated from the mains is irrelevant.

My reply wasn't aimed at you, but someone else who seemed to have mixed up ground as in the earth, with 0V and floating, as in not being connected to the soil, with a floating regulator, such as the LM317. I've highlighted the part where I believe the confusion occurred.

Attached is the schematic promised in Answer #41 that shows simplified circuits for a 'conventional' ground referenced PSU and a floating PSU. In both cases the feedback is shown.
(...)

I may have missed something, but the OPA192 has limited current output capability so, a PSU yes, but with something like 50 mA max. probably... if it's not further buffered. Just a thought!

No, you have not missed anything. Your observation about the current limitation is correct. There are missing functions too, like decoupling, compensation, and current limiting. But the purpose of the schematics is to illustrate the two feedback techniques which the OP was asking about. If I did a full design you would not be able to see the wood for the trees.

Just for a bit of fun, I did have a go at a 1A version with current limiting and all the twiddly bits. 

As far as doing a rough simulation of the LM317 is concerned. I prefer to use the generic op-amp model, as its parameters can be edited and is faster to simulate. Here's my basic LM317 simulation. I added in the current limiting but didn't include safe operating area protection. I agree this is only to show the basic concept and not a practical design.

[spec]

Ahh apologies H99. I did not get your point.